1. Field of the Invention
The present invention relates to a silicone composite product of which the inside part comprises a silicone gel and the surface layer comprises a silicone elastomer or silicone resin, and to a process for preparing the silicone composite product.
2. Description of the Prior Art
Cured silicone products extremely low in hardness and penetration as compared with elastomer are called "silicone gels" in the field of art. The silicone gels, because of their excellent electrically insulating properties and stability of electrical properties as well as excellent pliability, are used for potting or sealing of electric and electronic parts, especially as a coating material for covering control circuit components, such as power transistors, integrated circuits, capacitors, etc. to protect these components against thermal or mechanical disorder or malfunction. However, the surface of a silicone gel is so tacky as to permit easy adhesion of refuse or dust thereto and to cause such a workability problem that molded products cannot be superposed on the silicone gel. Furthermore, the silicone gel is unsatisfactory in mechanical strength, and may be broken or damaged in an operation for removal of the dust or the like adhered thereto.
As a method of improving the above-mentioned drawbacks of the silicone gels, there have been known, for example:
(i) a method in which a silicone gel is protected by a casing, a cover or the like formed of an organic material, thereby preventing the adhesion of refuse, dust, etc. on the silicone gel and achieve mechanical protection of the silicone gel;
(ii) a method in which a protective layer of other organic high-hardness resin is provided on a silicone gel;
(iii) a method which, as disclosed in Japanese Patent Publication (KOKOKU) No. 1-25704 (1989), comprises the steps of applying an organohydrogenpolysiloxane or the like to a silicone gel, and diffusing the organohydrogenpolysiloxane or the like into the surface of the silicone gel, followed by heating to cure the organohydrogenpolysiloxane or the like, thereby forming a silicone elastomer or silicone resin layer; and
(iv) a method in which, as disclosed in Japanese Pre-examination Patent Publication (KOKAI) No. 61-277414 (1986), an organohydrogenpolysiloxane is applied to an uncured silicone gel composition, and, before diffusion occurs, heating is carried out to simultaneously cure both the inside silicone gel composition and the surface layer in which the organohydrogenpolysiloxane has been incorporated, whereby a silicone elastomer layer is formed on the surface of the silicone gel.
However, the method (i) above involves such problems as unsatisfactory adhesion between the silicone gel and the casing, due to rising of the gel into corners of the casing.
Also, the method (ii) has the problem of separation between layers being liable to occur because of insufficient adhesion between the layers.
In the method (iii), the kind and amount of the organohydrogenpolysiloxane having functional groups capable of reacting with the residual functional groups in the silicone gel must be so selected that the organohydrogenpolysiloxane will not remain uncured on the surface of the silicone gel, or there is need for cleaning or wiping after the cure.
According to the method (iv), the silicone gel composition is still in an uncured state when heating is conducted. Therefore, the organohydrogenpolysiloxane applied to the surface of the silicone gel composition is taken into the composition under the effect of convection. As a result, the surface layer may be cured nonuniformly or it may be impossible to obtain a favorable protective layer.